Conventional edge grip end effectors have wafer grip pads with included angles that create problems solved by this invention. Conventional distal rest pads often have an inside corner into which the edge of the wafer is urged by a rear pusher to mechanically retain the wafer on the end effector.
FIG. 1 illustrates an embodiment of a spatula-shaped end effector 10 of this invention for transferring semiconductor wafers, such as a wafer 12 (shown transparent to reveal underlying structures), to and from a wafer cassette (not shown). End effector 10 is adapted to receive and securely hold wafer 12 and transfer it to and from a cassette for processing. Wafers having diameters of less than 150 mm are typically spaced apart at a 4.76 mm ( 3/16 inch) pitch distance; 200 mm diameter wafers are typically spaced apart at a 6.35 mm (¼ inch) pitch distance; and 300 mm wafers are typically spaced apart at a 10 mm (0.394 inch) pitch distance.
In general, the end effector 10 enters a wafer cassette to retrieve a wafer 12. The end effector 10 is then finely positioned and actuated to grip a periphery 14 of wafer 12, remove wafer 12 from the cassette (not shown), and transfer wafer 12 to a processing station (not shown) for processing. End effector 10 may then, if necessary, reinsert wafer 12 into the cassette, release wafer 12, and withdraw from the cassette.
End effector 10 is operably coupled to robot arm (not shown) at a proximal end 20 and extends to a distal end 22. End effector 10 receives wafer 12 between proximal end 20 and distal end 22 and includes at least two and, preferably, four rest pads upon which wafer 12 is initially loaded. Two distal rest pads are located at, or adjacent to, distal end 22 of end effector 10; and at least one, but preferably two proximal rest pads are located toward proximal end 20.
Semiconductor wafers have an annular exclusion zone, or inactive portion, that extends inwardly about 1 mm to about 5 mm from periphery 14 and completely surrounding wafer 12. Exclusion zone 30 is described as part of an industry standard wafer edge profile template in Semiconductor Equipment and Materials International (“SEMI”) specification M10298, pages 18 and 19. As a general rule, no part of end effector 10 may contact wafer 12 beyond the inner boundary of exclusion zone 30. The distance between distal rest pads and the distance between proximal rest pads each have an angular extent greater than any feature on wafer 12 to guarantee that wafer 12 is gripped only within exclusion zone 30.
Conventional rest pad designs often use overhung or sloped tip pads having an included angle between them. An end effector must move forward while also moving down so that the drop off trajectory is sloped greater than the tip angle. Accordingly, the final resting position of the wafer on the end effector is effected by variations in the height of the wafer during drop off. The end effector must decelerate in a horizontal direction at the time of drop off that will result in the wafer skidding on the stationary wafer supports. If the wafer is dropped off quickly (as is desirable for high throughput), the end effector will bounce chaotically during drop off due to the cantilever and changes in acceleration over this short motion. Coordinating the vertical and horizontal motions of the end effector will therefore vary from wafer to wafer at the exact moment of contact between the wafer and the stationary supports. Requiring horizontal motion to pick up or drop off a wafer effects the repeatability of the final drop off and the speed at which the drop off can occur for a given repeatability requirement.
FIG. 3 shows a substantially flat embodiment of a conventional distal rest pad 24. In this embodiment, the distal rest pad 24 includes a pad portion 32 and a backstop portion 34. The pad portion 32 is substantially parallel to an imaginary plane 36 extending through wafer 12. The backstop portion 34 is inclined toward wafer 12 at a backstop angle 38 relative to a line perpendicular to plane 36. Pad portion 32 has a length 40 that is a function of the depth of exclusion zone 30. The wafer 12 typically has a substantially rounded peripheral edge and contacts rest pads 24 only within exclusion zone 30. The wafer 12 is gripped by urging it into the included: angle formed between pad portion 32 and backstop portion 34.
FIG. 4 shows an inclined embodiment of a conventional distal rest pad 24. The distal rest pad 24 includes an inclined pad portion 42 and a backstop portion 34. In this embodiment, the inclined pad portion 42 is inclined away from wafer 12 at a rest pad angle 44. The backstop portion 34 is inclined toward wafer 12 at backstop angle 38. The inclined pad portion 42 has a length 40 that is a function of the depth of exclusion zone 30. Similar to the FIG. 3 embodiment, the wafer 12 typically has a substantially rounded peripheral edge and contacts rest pads 24 only within exclusion zone 30. The wafer 12 is gripped by urging it into the included angle formed between pad portion 42 and backstop portion 34. In the inclined embodiment, there is substantially no contact between rest pad 24 and a bottom surface 46 of wafer 12. Both the flat and inclined embodiments of distal rest pads 24 have a height 48 that substantially reaches but does not extend beyond the top surface of wafer 12.
There are several problems with the conventional distal rest pad designs shown in FIGS. 3-4. Among other things, it is difficult to release and place the wafer precisely and cleanly on the end effector because the wafer must move horizontally (as opposed to purely vertically) relative to the grip pads to be released from the conventional distal rest pad. In addition, the included angles formed between the contact surfaces of the conventional distal rest pad create particle traps that could lead to wafer contamination. Further, the pads support a wafer in an indeterminate position due to variations in the edge shape of the wafer and difficulty in manufacturing consistent internal angles on the rest pads. Depending on the specific geometry and coefficient of friction of the distal rest pads, it may be impossible to fully grip certain “sticky” wafers regardless of the pusher force.
What is needed, therefore, is a low-profile distal rest pad that eliminates the particle trapping characteristics of conventional rest pads, reduces the overall height of the support plate and may be manufactured from more than one material. The present invention provides, among other things, all of these features.